1. Field of the Invention
The present invention relates to microfluidic devices. In particular, the present invention relates to the use of magnetohydrodynamics to propel or mix fluids within microfluidic structures.
2. Prior Art
The field of microfluidics is growing rapidly. There is a strong desire to miniaturize chemical assays. A number of various technologies are currently being developed in an effort to develop what has become known as lab-on-a-chip (LOC) technology. It is believed that these technologies will lead to mobile, small scale chemical testing devices. Such devices would have a variety of applications. Emergency Medical Technicians and military medics could use such devices to rapidly analyze a person's blood chemistry. Forensic scientists could perform DNA analysis at a crime scene, instead of waiting hours or days for results from a laboratory. Realizing the great potential of such devices, there have been many attempts to find a low power method of accurately propelling extremely small liquid samples through microfluidic structures. The desired width of these channels is less than 1 mm, preferably 500 micrometers or less, preferably 100 micrometers or less.
Some of the characteristics sought in a microfluidic propulsion system include high fluid flow rates, the ability to change direction of the flow, minimal power requirements and the ability to effectively propel a wide variety of fluids through structures composed of a wide variety of solid materials. High fluid flow becomes more difficult as microfluidic structures become smaller. This is due to increased drag created by moving along the walls of the microfluidic structure. A small power requirement is desired so that devices may be compact and portable. Different microfluidic technologies have advantages and disadvantages in these areas.
It has been found that by forming a gradient of a hydrophobic film across a glass or silica plate, water droplets can be induced to travel along the gradient. However, this method has only achieved relatively slow flow rates. In addition it is difficult to scale down to the microfluidic level of less than 500 micrometers. Hydrophobic films tend to work best on relatively large water droplets. It is impossible to change flow direction and is only effective on aqueous solutions.
There has been some experimentation in using temperature gradients to propel water through small channels. Although flow is reversible, flow rate is very slow. This technique also requires a relatively large power supply.
Electrokinetics has been a popular field of study in microfluidics. It provides for easy change of flow direction and is suitable for very small channels. It is also well suited for separating chemicals. However, electrokinetics suffers from disadvantages. It is very sensitive to the chemical properties of both the fluid being manipulated and the walls of the channel. In addition, this technology requires high voltage and can only achieve relatively slow flow rates. Electrokinetics also will not work in the presence of air bubbles, which are common in microfluidic systems. Another disadvantage is that electrokinetics is ineffective on organic fluids. Like hydrophobic films, this method only works well on aqueous solutions. Application of a strong current may also alter chemicals present in the solution, thereby decreasing the accuracy of any analyses.
Mechanical methods of pumping fluids through microstructures also poses several problems. The mechanical methods usually require valves which can complicate fabrication and become clogged. Complex mechanical devices, including many valves, are difficult to scale down to small sizes. In addition, mechanical pumping usually requires a pulsating flow and it does not conveniently allow changes in flow direction.
Centrifugation is inexpensive and adaptable to a wide range of channel sizes. However, the flow direction cannot be reversed and this process usually involves a single-use cartridge. Centrifugation also requires a large power supply. These power requirements rapidly increase and the microfluidic structure size decreases due to drag.
There is a need for alternative non-mechanical pumping systems that are lower power, operate with a wider range of device materials and solutions compositions, offer multi-use capabilities, and allow easy change in flow direction.
Magnetohydrodynamics (MHD) has been proposed as an alternative method for microfluidic propulsion. This technology involves the application of a magnetic field and an electric field. The two fields are applied perpendicular to each other and perpendicular to the desired direction of flow. These fields induce fluid flow perpendicular to both fields. This is known as a Lorentz force. On larger scales, the Lorentz force is too weak for any practical applications and until recently has been considered only a curious phenomenon.
MHD works best when current density is high, and most electrodes have fairly low current density. However, because of the physics unique to small scale diffusion, microelectrodes exhibit very high current densities. MHD is therefore much more practical at very small scales. Relatively little power, less than one volt, can achieve high flow rates in microfluidic structures.
MHD is very susceptible to change of flow direction. By simply alternating the electrodes, the direction of fluid flow reverses. Similarly, reversing the magnetic field will also reverse flow direction. The ease of change in flow direction coupled with low power and high flow rate make MHD an excellent mechanism for microfluidic propulsion. In addition, Lorentz forces apply to all fluids, so that MHD may effectively propel both aqueous and organic solutions. MHD is also unaffected by the materials used to construct microfluidic structures.
There have been limited attempts to apply MHD technology to microfluidics already. It has been used successfully on molten metals and mercury. However, these generally involve high temperatures and are not well suited to be used in conjunction with chemical assays. These methods have high power requirements and chemical assays are generally not designed to utilize molten metals.
More recently, attempts have been made to apply MHD to aqueous solutions. Channels have been constructed having electrodes on opposing walls. A magnetic field is then applied perpendicular to both the direction of flow and the electric field generated by the electrodes. Unfortunately, a significant problem has arisen due to water electrolysis. Although insignificant on larger scales, bubbles formed by water electrolysis within a microstructure pose serious problems. Aside from blocking fluid flow, they also disrupt the electric field. This in turn disrupts the Lorentz forces and halts fluid flow completely. Only very low voltage, which results in very slow flow rates, have been shown to be practical. At higher voltages, water electrolysis makes MHD impossible. In addition, MHD is ineffective when applied to hydrophobic, oily solutions that have dielectric points greater than that of water.
There have been attempts to use an alternating current in conjunction with a synchronous alternating magnetic field to counteract the electrolysis of water. By constantly reversing the fields, bubble formation is reduced. Unfortunately, this only provides for a minimal increase in voltage and flow before electrolysis occurs. In addition it is much more difficult to perform. It requires precise shifts in the electric and magnetic fields, otherwise the fluid does not flow at all.
It is therefore desirable to provide a microfluidic propulsion technique that requires relatively little power.
It is also desirable to provide a microfluidic propulsion technique that utilizes a constant magnetic field.
It is also desirable to provide a microfluidic propulsion technique that may be used on a variety of fluids, specifically aqueous and hydrophobic solutions and structures.
It is also desirable to provide a microfluidic propulsion technique that does not induce water electrolysis.